1. Field of the Invention
The present invention relates to a communication circuit system, more particularly relates to a voltage drop circuit, an oscillation circuit, and a wave detection circuit suitable for use in a non-contact type IC card etc. carrying a large-scale integrated circuit (LSI).
2. Description of the Related Art
An IC card comprises a microprocessor, that is, the central control portion and computation portion of a microcomputer, housed in an LSI, and an IC memory which are built into a single silicon substrate. It has multiple purposes and can be used for multiple functions and is difficult to steal or forge, so is expected to spread rapidly in use.
There are two types of IC cards: a contact type wherein the connection with external apparatuses is performed through electroconductive terminals provided in the card, and a non-contact type wherein the contact portion is made non-contact in type and data is communicated using electromagnetic waves, light, etc.
A contact-type IC card suffers from poor contact due to wear of the contacts, dirt, etc. and therefore an occasional inability to send or receive data. The non-contact type IC card is free from such trouble and has the advantage of enabling excellent data transmission and reception at relatively close distances.
A non-contact type IC card generally houses a battery and uses the voltage of the same to operate an oscillation circuit, control circuit, storage unit, etc. in the card. For example, it is constituted so that the control circuit outputs queried (inquiry) information etc. stored in the storage unit based on reference signals from the oscillation circuit.
As the microprocessor mounted inside an IC card, generally use is made of a low power consumption CMOS microprocessor. In recent years, however, due to the need to extend the usage life of the internal battery, it has been desired to reduce the voltage of the power source to an extremely low level. Also, due to need for competition with magnetic cards and other existing media, it has been desired to reduce costs.
As the oscillation circuit built in an IC card LSI, use is made of a CR self-oscillation circuit. In the past, this CR oscillation circuit has been designed to suppress the so-called "punch-through current" with respect to a smooth input waveform by the provision of a differential comparator at the first stage of the drive circuit. Further, it has been desired that the IC card oscillation circuit have a current consumption of at least as low as about 0.7 .mu.A.
In general, the current consumption of a CMOS circuit is expressed by the sum of the punch-through current at the time of switching and the charging and discharging current of the load capacity, but in a conventional IC card LSI as mentioned above the reduction of the voltage of the power source has led to the problem that when a certain fixed low voltage, for example, 1.5 V, is dropped to, manufacturing variations in the threshold voltage of the MOS transistors cause a decline in the operating speed and in extreme cases the output being fixed at either of the high level or low level and thus an inability of operation.
To solve this problem, new manufacturing processes are being developed which lower the threshold voltage V.sub.TH and hold down the variations to smaller than in the past, but since these are advanced processes, one cannot expect to achieve the above-mentioned target of reduced costs with them.
Further, the above-mentioned conventional oscillation circuit suppresses the punch-through current by providing a differential comparator at the first stage of the drive circuit, but the differential comparator requires a direct current of about 0.1 .mu.A for each of a bias current of the differential input portion and the bias reference generating circuit and the comparator reference voltage generating circuit, and in principle the duty ratio is not even 50 percent.
As a result, since it is necessary to cause oscillation once at twice the frequency and to divide the same, the current consumption for the oscillation circuit as a whole ends up becoming 1.5 .mu.A, which poses a problem in use for an IC card.
Further, a non-contact IC card using electromagnetic waves performs the reading and writing of data by microwaves, so requires a circuit for detecting waves at a high sensitivity and with a low power consumption.
various circuits have been proposed for this wave detection circuit.
Among these circuits, as one which has a power consumption of less than 10 .mu.W, there is known a diode wave detection circuit which in principle consumes no power.
The above-mentioned diode wave detection circuit, however, requires an input amplitude of the threshold voltage V.sub.TH of the transistor, which is, for example, at least 0.7 V, so it is difficult to detect waves with a high sensitivity.
Typically, the circuits applied to the IC card are described, however, these problems may arise such circuits applied devices other than the IC card.